1. Field of the Invention
The present invention relates to shift control apparatus and method for a hybrid transmission suitable for use in a hybrid vehicle in which a main power source such as an engine and motor/generators are mounted. The present invention particularly relates to the shift control apparatus and method which are capable of having a differential equipment (unit) interposed between the main power source and the motor/generators carry out a continuously variable shift operation.
2. Description of the Related Art
Such a kind of hybrid transmission as described above, for example, includes a differential unit having two degrees of freedom and which is constituted by a planetary gear group or so forth. An input from the engine which is the main power source, an output to a drive system, and a plurality of motor/generators are mutually coupled to respective revolutional members of the differential unit so that powers from the motor/generators permit the continuously variable shift operation. In the above-described hybrid transmission, the motor/generators are driven by means of an electric power of a battery. However, when this driving is carried out, it is necessary for the motor/generators to be driven at a power equal to or lower than a power rating of the battery in the same case as the drive for a normally available electrical equipment.
A Japanese Patent Application First Publication No. Heisei 9-191506 published on Jul. 22, 1997 exemplifies a previously proposed technique controlling the drive torque to the motor/generators (function as motors) in accordance with a state of the battery. In the previously proposed technique disclosed in the above-identified Japanese Patent Application First Publication, an electric vehicle in which a motor is driven along with a charge-and-discharge of the battery which serves as a power source is prerequisite. When a state variable of the battery is varied by a reference value or more such as reductions in a battery voltage or a battery residual capacity equal to or lower than a reference value, a response speed of a torque control for a drive torque command to the motor is slowed so that an earlier deterioration of the battery is prevented.